1. Field of the Invention
The present invention relates to a method of heating a plated steel plate like a metal plated steel plate such as a tin plated steel plate, a zinc plated steel and others by directly applying a current to the plated steel plate. The heating method of the present invention is used in hot press processing of the plated steel plate, for example.
2. Description of the Related Art
Various heating methods are used when performing hot press processing on a plated steel plate. One of the methods is a method of heating the plated steel plate by directly applying a current to the plated steel plate. This method is called resistance heating or direct current application heating. This method uses the phenomenon that the plated steel plate generates heat in proportion to an electrical resistance thereof when the current is applied. This method has an advantage in that power can be converted to heat efficiently, and that the plated steel plate can be heated to a hardening temperature in a short time. The current applied to the plated steel plate may be either a direct current or an alternating current. Since commercial power is generally used as is, the alternating current is often used. When using the alternating current, increase in the electrical resistance by a skin effect is anticipated if the plated steel plate is thick, and hence the plated steel plate can be more efficiently heated.
When a current is directly applied to the plated steel plate to perform the hot press processing, the plating temporarily melts and becomes biased. For example, when a current is applied in the extending direction of the plated steel plate having a rectangular shape in plain view, the melted plating biases toward the middle in the width direction and heaps, and in contrast, the plating near the side surface becomes thin. The side surface as referred to herein is the end face parallel to the thickness direction and the extending direction of the plated steel plate. Since the plating prevents generation of a scale (oxide layer) on the surface of the heated plated steel plate or prevents generation of a rust after the hot press processing, when the plating is biased, the generation of the scale may not be prevented, or the rust preventing performance after the hot press processing may be impaired.
For example, paragraph [0007] of Japanese Unexamined Patent Publication No. 2010-070800 describes that when a large current is directly applied on the plated steel plate, an attracting force based on the Fleming's left hand rule is generated with respect to the melted plating by the current and the magnetic field generated by the current, and the attracting force becomes the cause of bias of the plating. In the invention of this publication, the current density is made small when the plating is thick whereas the current density is made large when the plating is thin to prevent the bias in the plating. Specifically, the current having a current density that satisfies the mathematical equation I≦(23−T)/0.0718 described in claim 1 is applied. In this equation, I is the current density (A/mm2) and T is the thickness (μm) of the plated layer. Furthermore, paragraph [0019] describes that when the thickness of the plating exceeds 22 μm, the amount of current flowed through the plated layer becomes large even if the current density is made small, whereby the attractive force based on the Fleming's left hand rule becomes large and the plating biases. Moreover, paragraph [0014] describes that the plating and the steel plate are rapidly alloyed when the thickness of the plating is sufficiently small, and hence the bias in the plating is effectively prevented.
In the invention of the above publication, the thickness of the plating needs to be within the range of greater than or equal to 2 μm and smaller than or equal to 22 μm, but a strict management of the film thickness is cumbersome. Moreover, there is a possibility that the scale may be generated or the rust may be produced at the time of the hot press processing when the film thickness is small. Furthermore, the electrical plating method, whose cost is high, is inevitably adopted to make the film thickness small and to strictly manage the film thickness, and hence the hot dip plating method, whose cost is low, is difficult to adopt.
It is an object of the present invention to provide a method of heating a plated steel plate capable of easily and conveniently resolving the bias in the plated layer without relying on the strict management of the film thickness and the control of the current density.